A rate gyroscope can be used to determine the direction and magnitude of a turn performed by a land vehicle through the process of integrating the indicated rate from the initial position to the current time. In order to determine the actual vehicle azimuth angle (with respect to true North or other reference), the initial vehicle azimuth must be known. Ordinarily one can assume that the initial vehicle azimuth is the same as the final vehicle azimuth for the last trip. Exceptions are when the vehicle has been rotated on a turntable, is transported on a ferry or on another vehicle, battery power has been lost, or other event such as a repair has invalidated the assumption that the vehicle has remained stationary at the last known azimuth, or the azimuth information has been lost.
Absolute azimuth angle measurement instruments such as the magnetic compass have been used, but are subject to large and unpredictable errors associated with magnetic anomalies, bridges, tunnels, power lines, and other ferrous metal objects along the route. Gyro compasses are too expensive for many applications. Other aids to land navigation such as beacons and the global positioning system (GPS) do not intrinsicly provide information on the actual vehicle azimuth.
Some urban mass transit and some automobile navigation systems provide for vehicle navigation and reporting through the use of "signposts", which are typically radio or optical beacons installed at periodic intervals along the route. The position of the vehicle in transit between the signposts is obtained through dead reckoning (DR) navigation. This technique combines knowledge of the vehicle azimuth and distance travelled (typically obtained from the vehicle odometer) to compute the geographic position of the vehicle. Azimuth information is typically provided by a magnetic compass, and is subject to the anomalies described previously. These errors make it difficult to determine promptly whether the vehicle has left the desired route or to compute the vehicle location in these circumstances. A more accurate method of measuring the vehicle azimuth is the integrated output of a rate gyro, or the angle output of a rate integrating gyro, but the initial angle must be known. As mentioned previously, determining the initial vehicle azimuth is generally not a problem, but it must occasionally be re-established due to special circumstances, and a need exists for establishing an initial vehicle azimuth soon after the previous vehicle azimuth data is invalidated.